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Abstract

We report the observation of room-temperature optical gain at 1.3 μm in electrically
driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is
calculated with respect to injected power for samples with and without a confinement
aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples.
At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated
power levels, the sample with confinement aperture gives slightly higher gain.

Keywords:

Background

InxGa1-xAs1-yNy semiconductor alloy was first proposed by Kondow et al. in 1996 [1], and considerable research attention has been devoted to this alloy system due to
its possible optoelectronic applications at an operating wavelength of 1.3 μm. With
the addition of small amounts of nitrogen into the (In)GaAs lattice, a strong electron
confinement and bandgap reduction are obtained. Furthermore, addition of N allows
band engineering, allowing the device operating wavelength range to extend up to 1.6 μm
[2]. An extensive set of different devices based on this alloy has been fabricated and
demonstrated [3]. Examples of these devices are vertical cavity surface-emitting lasers (VCSELs) [4-6], vertical external cavity surface-emitting lasers [7,8], solar cells [8,9], edge-emitting lasers [10], photodetectors [11], semiconductor optical amplifiers (SOAs) [12], and vertical cavity semiconductor optical amplifiers (VCSOAs) [13,14].

VCSOAs can be seen as the natural evolution of SOAs, which, owing to their fast response,
reduced size, and low-threshold nonlinear behavior, are popular in applications such
as optical routing, signal regeneration, and wavelength shifting. Within these fields,
VCSOAs have been used as optical preamplifiers, switches, and interconnects [15-17]. Their geometry provides numerous advantages over the edge-emitting counterpart SOAs,
including low noise figure, circular emission, polarization insensitivity, possibility
to build high-density two-dimensional arrays of devices that are easy to test on wafer,
and low-power consumption that is instrumental for high-density photonic integrated
circuits. Generally speaking, a VCSOA is a modified version of a VCSEL that is driven
below lasing threshold. The first experimental study of an InxGa1-xAs1-yNy/GaAs-based VCSOA was reported in 2002 [18], with a theoretical analysis published in 2004 [19]. Several studies on optically pumped InxGa1-xAs1-yNy VCSOAs have been published [14,20-23], while electrically driven VCSOAs have been demonstrated only in ‘Hellish’ configuration
[24]. The present contribution builds on these technological developments to focus on
an electrically driven multifunction standard VCSOA device operating in the 1.3-μm
wavelength window.

Methods

The amplification properties of InxGa1-xAs1-yNy VCSOAs were studied using a 1,265- to 1,345-nm tunable laser (TL; TLM-8700-H-O, Newport
Corporation, Irvine, CA, USA), whose output was sent to the sample using the setup
shown in Figure 1a. The TL signal was split via a 10/90 coupler to a power meter and to the sample,
respectively. Back reflections were avoided using an optical isolator while the TL
power was changed from 0 to 7 mW using an optical attenuator. A lens-ended fiber (SMF-28
fiber, conical lens with cone angle of 80° to 90° and radius of 6.0 ±1.0 μm) was used
to focus the TL light to the sample surface as well as to collect its reflected/emitted/amplified
light, which was then directed to an optical spectrum analyzer (OSA). The VCSOA was
electrically DC biased up to 10 mA and stabilized in temperature at 20°C via a Peltier
cooler.

Figure 1.Experimental setup (a) and the layer structure of the investigated samples (b).

The investigated VCSOA structure with a 3.5λ cavity is shown in Figure 1b. The structures were grown by a solid source molecular beam epitaxy reactor with
a radio frequency plasma source for incorporating nitrogen. The growth was carried
on an n-type GaAs(100) substrate, and the bottom and top distributed Bragg reflectors
(DBRs) were doped with silicon (n-type) and beryllium (p-type), respectively. The
two DBRs comprised 21 and 24 pairs of AlxGa1-xAs/GaAs layers for the top and bottom DBR, respectively. The Al concentrations were
x = 0.8 and 0.98 in the top and bottom DBRs, respectively. The confinement aperture,
which is required for better carrier and light confinement, was defined in the uppermost
layer of the bottom DBR. The active region contains three stacks of three 7-nm-thick
In0.35Ga0.65As0.975 N0.025 quantum wells separated by 20-nm thick GaAs spacers. A set of several VCSOA samples
was fabricated, having different dimensions of the top DBR mirror radius (R1), confinement aperture radius (R2), and bottom DBR radius (R3) for cases with and without the confinement aperture. In this paper, we compare the
results obtained for two samples with and without confinement aperture, with R1 = 5 μm, R2 = 25 μm, and R3 = 50 μm.

Results and discussion

Room-temperature reflectivity and photoluminescence (PL) measurements were performed
on the as-grown sample, and the results are shown in Figure 2. Simulated reflection is also shown in the figure. Two resonances λR1 and λR2 are observed within the DBR stop band as a result of the relatively long cavity length
[25]. The principle resonance, which is designed for 1.3-μm operation, is observed at
λR1 = 1,282 nm, while the other unwanted resonance at lower wavelength is observed at
λR2 = 1,235 nm. Figure 3 shows the VCSOA amplified spontaneous emission (ASE) spectra obtained with no optical
injection at different applied bias currents of 0 to 10 mA for the sample without
confinement aperture. The highest ASE power peak appears at 1,288 nm and is blue-shifted
with respect to that of the lasing cavity mode wavelength [26,27]. The other modes are also consistent with the PL spectra. Figure 3 shows that with increasing the bias current, the amplitude of each mode increases
and also slightly shifts towards higher wavelengths. This shift is associated with
local temperature increase in the device. A similar result was observed in the VCSOA
with the confinement aperture.

Figure 2.Room temperature photoluminescence (red) and reflectance spectra of the studied structure. Experimental and simulated reflectivity spectra of the studied VCSOA structure are
shown in black and blue lines, respectively.

Figure 3.Power spectra of VCSOA without confinement aperture obtained for different bias currents.

Since no significant change in the spectrum amplitude above 7 mA was observed, we
investigated the devices up to this current value. ASE power spectra taken at 7 and
0 mA (ASE0) are used to extract the spectrum ASE - ASE0, which is the spectrum where the background noise is minimized. To find the amplified
optical signal (AOS), we injected light sweeping the TL wavelength (λinj) from 1,266 to 1,310 nm with a 7-mA current bias. Figure 4 shows results for injection at λinj =1,279 nm only. We could not investigate the second resonance peak λR2 because of the wavelength limit of the TL. In Figure 4a,b,c,d, the results for ASE - ASE0, AOS + ASE, AOS + ASE - ASE0, and finally AOS - ASE0spectra are shown, respectively.

As the gain is small, the amplified signal cannot be easily discerned in Figure 4d. Hence, the gain was calculated using the simple relation

(1)

for each wavelength after obtaining AOS and ASE data. Results are shown as a function
of the injected wavelength in Figure 5 for a specific laser power (Pinj) of 2.25 nW. A maximum gain of 3 dB with a very broad peak is observed at the maximum
ASE wavelength of 1,288.5 nm. In the study, measured signal levels are very near to
limits of the OSA; therefore, larger bandwidth wavelength values are used, which can
be the reason of the broadness of the gain peak.

Having verified that the gain peak corresponds to the ASE peak wavelength, we investigated
the Pinj dependence by varying it from 1.5 nW to a few milliwatts for the single wavelength
of 1,288.5 nm. Results are presented for both samples with and without confinement
aperture in Figure 6 for power values below 10 nW. For injected laser powers over 5 nW, the gain falls
rapidly. At the lowest injected power, the sample with confinement aperture exhibits
10 dB of gain, which is observed near the maximum ASE wavelength. For the investigated
injected power range, the sample with the confinement aperture showed a higher gain
because of the better carrier and light confinement in the VCSOA.

Figure 6.Power-dependent gain for the samples with and without confinement aperture.

Conclusions

In this paper, we report the observation of gain in an electrically driven dilute
nitride VCSOA device operated at 1.3-μm in reflection mode. Two different types of
samples with and without confinement aperture are investigated. The ASE power peak
is found to be at 1,288.5 nm with additional modes, which are caused by the length
of the cavity. Optical gain is found to occur at low optical injection values. Above
5 nW of optical injection, the gain is found to fall rapidly. The maximum observed
optical gain is observed at 1,288.5 nm at room temperature. The maximum observed optical
gain at 7-mA current at room temperature is around 10 and 6 dB for samples with and
without confinement aperture, respectively. It is important to mention that despite
the small gain, the device is very promising because it requires very small currents
compared with in-plane SOAs.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

SBL, NAK, and SM carried out the measurements and data analysis. VMK and MG performed
the growth of structures. GM and MS carried the fabrication of devices. MJA performed
the theoretical studies and analysis. NB is the project leader. SBL and NB wrote the
paper. All authors read, corrected, and approved the final manuscript.

Acknowledgements

Support from EPSRC under grant EP/G023972/1 is gratefully acknowledged. Sefer Bora
Lisesivdin also acknowledges partial support from the Turkish Scientific and Technological
Research Council (TUBITAK) 2219 coded scholarship. COST Action MP0805 is also gratefully
acknowledged.